=64. Testing apparatus.= The arrangement shown in Fig. 17 is very convenient for many measurements in radio-activity. Two parallel insulated metal plates _A_ and _B_ are placed inside a metal vessel _V_, provided with a side door. The plate _A_ is connected with one terminal of a battery of small storage cells, the other pole of which is earthed; the plate _B_ with the electrometer, and the vessel _V_ with earth. The shaded areas in the figure indicate the position of ebonite insulators. The active material to be tested is spread uniformly in a shallow groove (about 5 cms. square and 2 mms. deep) in the brass plate _A_. In order to avoid breaking the battery connection every time the plate _A_ is removed, the wire from the battery is permanently connected with the metal block _N_ resting on the ebonite support. In this arrangement there is no possibility of a conduction leak from the plate _A_ to _B_, since the earth-connected vessel _V_ intervenes. [Illustration: Fig. 17.] An apparatus of this kind is very convenient for testing the absorption of the radiations by solid screens, as well as for making comparative studies of the activity of different bodies. Unless very active preparations of radium are employed, a battery of 300 volts is sufficient to ensure saturation when the plates are not more than 5 centimetres apart. If substances which give off a radio-active emanation are being tested, the effect of the emanation can be eliminated by passing a steady current of air from a gas bag between the plates. This removes the emanation as fast as it is produced. If a clean plate is put in the place of _A_, a small movement of the electrometer needle is always observed. If there is no radio-active substance in the neighbourhood, this effect is due to the small natural ionization of the air. We can correct for this natural leak when necessary. =65.= We have often to measure the activity due to the emanations of thorium or radium, or the excited activity produced by those emanations on rods or wires. A convenient apparatus for this purpose is shown in Fig. 18. The cylinder _B_ is connected with the battery in the usual way, and the central conductor _A_ with the electrometer. This central rod is insulated from the external cylinder by an ebonite cork, which is divided into two parts by a metal ring _CC´_ connected to earth. This ring acts the part of a guard-ring, and prevents any conduction leak between _B_ and _A_. The ebonite is thus only required to insulate satisfactorily for the small rise of potential produced on _A_ during the experiment. In all accurate measurements of current in radio-activity the guard-ring principle should always be used to ensure good insulation. This is easily secured when the ebonite is only required to insulate for a fraction of a volt, instead of for several hundred volts, as is the case when the guard-ring is absent. [Illustration: Fig. 18.]